Range selector question/problem

Discussion in 'Heavy Duty Diesel Truck Mechanics Forum' started by God prefers Diesels, Mar 8, 2021.

  1. God prefers Diesels

    God prefers Diesels Road Train Member

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    FRF-15210b

    Ever since I've owned this truck, maybe once a week it would grind from high to low. I figured the syncro was going out. But this week, it started doing something different. When I go to low range, it won't be in any gear at all. I'm physically in fifth gear, but the engine is idling. Then all the sudden the range selector will do it's thing, and the truck will "slam" into fifth gear. Kind of like if you were in fifth at 1500rpm, pushed in the clutch until the engine went to idle, and then popped the clutch back out.

    You still think syncro, or could the two issues together point to something else? Thanks.

    Edit: When I'm paying attention and catch it, I can move the tranny back to neutral, and move the range selector up and back down, and it will work fine then.
     
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  3. Dino soar

    Dino soar Road Train Member

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    There is a slave valve on the side of the transmission they can go bad and make it hard to shift from low to high range.

    If you have any air leaks in the lines or at that valve that also will make it sluggish and not work right. Check the heavy lines that run from the back of the transmission up to the slave valve and make sure you don't have any leaks in the small lines to the shifter.

    On the back end of the transmission I think it's called a regulator but it's actually a filter if I remember correctly and if your air can't freely go through there that also will cause you shifting problems.

    And if you have any moisture going through your system and it's cold that will make everything worse.

    Other than that everything else is in the back end of the transmission but it's common for air leaks or that slave valve or the regulator to be problems.

    And the range selector itself is the last part of that because it has to divert the air properly when you flip it. I suppose it is possible they could go bad also.
     
  4. God prefers Diesels

    God prefers Diesels Road Train Member

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    I'll check that filter out. The "going into neutral" thing happened after I was up North in the cold. I just got home, and I'm determined to sit on my butt the rest of the day. But I'm going to start digging into it tomorrow. Thanks for the info. As I uncover stuff, I'll report back.
     
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  5. Inderjit

    Inderjit Light Load Member

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    What transmission is it?
     
  6. BoxCarKidd

    BoxCarKidd Road Train Member

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    In the case of your model I thank the slave valve ( might be called a control valve ) is on the top plate. Right and forward of the shift tower. It does the same function as the one on the side.
    The filter on the back is also a regulator. Should allow about 65 PSI.
    Believe you can take the back cover off the range air cylinder and see its condition. Have seen them dry and or gummed up.
    Hope you find something simple like Dino pointed out but sounds like a bad syncro.
    Eaton Fuller has good info on line.
     
  7. God prefers Diesels

    God prefers Diesels Road Train Member

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    "Air module assembly". Yeah, it's on the top right. I plan to pull it from the shifter hole in the cab. I'll take the regulator/filter off at the same time.

    Here are all the parts:
    Eaton Inforanger

    I'd like to rebuild everything at the same time, except changing the syncro. If the rebuilds don't fix the problem, I'll tackle that afterward. If anyone wants to look through there and tell me the kits I need to make this happen, so I can get it all at once and not miss anything, I'd appreciate it. I know you all have lives.
     
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  8. SmallPackage

    SmallPackage Road Train Member

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    Are you saying the shifter is in neutral but the trans stays in gear? If so Next time it does it just blip the throttle and it will go into neutral to work around it. If its not grinding or hard to get into gear it sounds like the filter is just gummed up. You will see oil dripping from those valves when they get a ton of miles on them. Was common on the old first generations of 13’s after they got a few years behind them.
     
  9. SmallPackage

    SmallPackage Road Train Member

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    Have shifter in neutral when blipping the throttle so the trans can go into neutral without slamming the gears. Then put shifter back into the gear you need and go. If that makes sense.
     
  10. Brettj3876

    Brettj3876 Road Train Member

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    We had the same problem with a mack 18sp. Ended up being synchro. Would only do it going from high to low range shift. Wouldn't bang all the time either. More like 2 out of every 5 range shifts it would catch and BANG

    One of the shift forks we replaced too. It had a few thousand burr on one side
     
  11. God prefers Diesels

    God prefers Diesels Road Train Member

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    No, the shifter is in gear. (fifth) But the whole situation is in neutral. Dropping from sixth to fifth, say I should be at 1500 rpm, but the engine is idling. Picture this - you're in a pickup or a car, and you're in fifth gear going down the interstate. Push in the clutch, put it in second gear, and pop the clutch out. That's what I mean by "slamming" into gear. So my truck is at idle in fifth, and then the range selector finally does it's thing, and you have that dramatic rise in RPM, and it feels like you just slammed on the brakes because the engine is immediately hitting 1500rpm from an idle.

    I understand what you're saying. Like blipping the throttle to split a 13-speed.

    On mine, the syncro grinding is still happening at the same pace. Once or twice a week, or whatever. Hasn't got any worse. But this thing with the range not going into gear is new this last couple days.
     
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